1. Field of the Invention
This invention relates to the field of current integration circuits.
2. Description of the Related Art
It is often necessary to know the magnitude of a particular current over time. This can be determined with a current integrator.
Current integrators are well-known; a basic implementation is shown in FIG. 1. An operational amplifier A1 receives the current to be integrated Iin at its inverting input, with its non-inverting input grounded. A fixed integration capacitor C is connected between the op amp""s output and inverting input. A switch SR is connected across capacitor C, which resets the integrator when closed. Input current Iin, is integrated on capacitor C to produce an output voltage Vout from A1.
This arrangement suffers a number of shortcomings, however. If Vmax is the maximum output voltage that A1 can produce, then the maximum charge Qmax that can be stored on integration capacitor C without causing A1""s output to become saturated is given by Qmax=Vmax*C. Thus, to achieve a high Qmax requires a large C value.
The minimum charge Qmin that can be detected is also often of interest. This is also largely determined by the value of C. A small C value gives the circuit a high integration gain; i.e., Vout changes quickly for a given input current. Thus, a small C value allows small charges to be detected, but also results in a small Qmax value. A large C value gives a lower integration gain (Vout changes more slowly for the same input current) and a larger Qmax value, but also increases the minimum charge Qmin that can be detected. These conflicting requirements act to narrow the range of input currents which can be accurately integrated with the FIG. 1 circuit.
One approach to solving this problem in shown in FIG. 2. An array of integration capacitors such as Ca, Cb and Cc are used to allow different integration gains to be selected, using respective switches Sa, Sb and Sc. However, this arrangement conventionally requires that the capacitors, and thus the integration gain, be selected before the input current is integrated. If the magnitude of the input current or charge is unknown, it is difficult to select the correct capacitance to provide an integration gain which maximizes the integrator""s signal-to-noise ratio.
An auto-ranging current integration circuit is presented which overcomes the problems noted above.
The present circuit includes an operational amplifier which receives an input current to be integrated. Initially, a first integration capacitor is connected between the op amp""s output and inverting input, which integrates the input current and causes the op amp""s output voltage to increase. Additional integration capacitors may be switchably connected in parallel with the first integration capacitor.
A control circuit operates the switches which select the additional integration capacitors. The control circuit is arranged to close one of the switches and thereby connect an additional integration capacitor in parallel with the first capacitor whenever the op amp""s output exceeds a predetermined reference voltage, but before the output becomes saturated. In this way, a small integration capacitance is automatically employed for a small input current, and larger capacitance values are automatically switched in for larger input currentsxe2x80x94which lowers the integration gain, prevents the output from saturating, and keeps the current integration circuit""s signal-to-noise ratio high.